Hitherto, profile extrusion molding is employed for producing building products such as window frames, gutters, etc. from plastics. Resins to be used in profile extrusion molding should have properties to follow complicated shapes of dies and have good dimensional accuracy. From the viewpoint of those properties and also costs, polyvinyl chloride resins and polyolefin resins are generally used. However, it is difficult to use polyolefin resins particularly in applications requiring adhesion since they usually have less adhesion to various adhesives, and also in applications requiring transparency since they are whitened by crystallization. Thus, vinyl chloride resins are mainly used. But, there is a trend to substitute the vinyl chloride resins with other materials because of various problems arising these years. Among various substitute materials, polyester is a promising material from the viewpoint of its physical properties, environmental friendliness, adhesion properties, costs, etc. However, there are some problems in substituting the vinyl chloride resins used in profile extrusion molding with polyester.
In general, a profile extrusion molding process starts from a kneading and extrusion step and proceeds in the sequence of a profile die step, a sizing step, a cooling step, a cutting step and a fabrication step. When a polyester resin is processed by such steps, it sags in the course from the profile die to the sizing step because of the insufficient melt strength of the resin. Therefore, the molded resin cannot be conveyed to the next step, or cannot maintain its adequate shape.
Here, the maintenance of a shape is explained. Different from usual extrusion molding, the shapes of profile shapes are very complicated and often they have a hollow portion. In addition, in many cases, the profile shapes have sharp corners or edges at open part margins. When a resin having insufficient melt strength is used, a drawback such that the corners or edges are rounded before the resin reaches the sizing step tends to appear.
In general, to avoid such a drawback, a multi-plate sizing system or a vacuum sizing system is used in the sizing step to forcibly fix the shape. With such a system, since the polymer is forcibly deformed while cooling, a residual stress remains in the product so that stress cracking is caused in the product with solvents, solvent vapors or quick change of temperature. To overcome such a problem, the further improvement of melt strength of the resins is required in comparison with resins used in usual extrusion processing.
To solve such a problem, several proposals have been made. For example, a method is proposed for improving the dimensional accuracy of a profile shape by using polyester having a high melt viscosity in a range from a low shear region to a high shear region to avoid the sagging of the resin during molding (for example, JP-A-9-290451). Although this method can improve the sagging of the resin, the molding of the resin becomes difficult at low temperature since the melt viscosity in the high shear region is too high. Thus, such a resin must be molded at high temperature. Therefore, the polyester is severely thermally decomposed, extrusion marks often appear, and weld lines heavily form, so that the color tone, shape, strength, etc. of the profile shapes are worsened. Thus, further improvement of the resins is sought.
Furthermore, another method is proposed for maintaining a melt strength of a polymer by branching the polymer to reduce a viscosity of the polymer in a high shear region of a die and then restoring the viscosity in a shear-free region after extrusion (WO 00/77096). Although this method can improve the melt strength and the sagging of the resin to some extent, the effects are not satisfactory, and thus the further improvement is desired.
In the case of a blow extrusion application, a reactive melt-strength intensifier having a weight average molecular weight of 1,000,000 to 4,000,000 is proposed as a reactive melt-strength improver, and the effect of the intensifier to improve the melt strength is shown (JP-B-3237913). That is, this patent proposes a transparent polyester resin composition by the addition of a melt-strength intensifier having the same refractive index as that of a resin to a transparent amorphous polyester resin. However, an amount of this type of the melt-strength intensifer and processing conditions should be completely adjusted, since a vinyl aromatic modifier having a high molecular weight should be dispersed in the polyester resin. For example, when the melt-strength intensifier is added in an amount necessary for achieving the intended melt strength, the processing conditions are limited in certain narrow ranges. If the processing conditions deviate from such narrow ranges, the melt strength easily changes to induce an insufficient melt strength or an excessive melt strength. Since the compatibility of the melt-strength intensifier and the amorphous polyester is probably low, voids may sometimes be formed by slight bending when a profile shape is fixed, for example, in the processing of the article, so that a part containing the voids is whitened. That is, when the melt-strength intensifier has an excessively large molecular weight, a length of an interface between the intensifier and the amorphous resin increases so that the size of this region increases to a visible region or larger, which is disadvantageous for whitening on bending.
In addition, since the above resin composition has low resistance to chemicals such as solvents, detergents, etc., it is whitened or swelled when it is wiped with the solvents or detergents in the processing step or usual cleaning, so that the appearance of the profile shape is deteriorated.